// the first week of the year is the one which contains Jan, 4 (according
// to ISO standard rule), so the year day N0 = 4 + 7*W always lies in the
// week W+1. As any day N = 7*W + 4 + (N - 4)%7, it lies in the same week
// the first week of the year is the one which contains Jan, 4 (according
// to ISO standard rule), so the year day N0 = 4 + 7*W always lies in the
// week W+1. As any day N = 7*W + 4 + (N - 4)%7, it lies in the same week